vintner: let MSBuild find the Windows SDK without the registry.

GetLatestSDKTargetPlatformVersion normally resolves the SDK via the
registry, which has nothing to find under Wine. Pass it an extra list of
roots (populated by vintner.props, see the ImportBefore directory next
to this file) so it can also look directly under the VS install root.

diff --git a/MSBuild/Microsoft/VC/v180/Microsoft.Cpp.WindowsSDK.props b/MSBuild/Microsoft/VC/v180/Microsoft.Cpp.WindowsSDK.props
index 5003a04..2a785a4 100644
--- a/MSBuild/Microsoft/VC/v180/Microsoft.Cpp.WindowsSDK.props
+++ b/MSBuild/Microsoft/VC/v180/Microsoft.Cpp.WindowsSDK.props
@@ -110,7 +110,7 @@ Copyright (C) Microsoft Corporation. All rights reserved.
     <SDKVersion Condition="'$(SDKVersion)' == ''">10.0</SDKVersion>
     <SDKDisplayName Condition="'$(SDKDisplayName)' == ''">Windows 10</SDKDisplayName>
 
-    <_LatestWindowsTargetPlatformVersion>$([Microsoft.Build.Utilities.ToolLocationHelper]::GetLatestSDKTargetPlatformVersion($(SDKIdentifier), $(SDKVersion)))</_LatestWindowsTargetPlatformVersion>
+    <_LatestWindowsTargetPlatformVersion>$([Microsoft.Build.Utilities.ToolLocationHelper]::GetLatestSDKTargetPlatformVersion($(SDKIdentifier), $(SDKVersion), $(MsvcGoWine_ExtraSdkRoots.Split(';'))))</_LatestWindowsTargetPlatformVersion>
     <WindowsTargetPlatformVersion Condition="'$(WindowsTargetPlatformVersion)' == '' and '$(_LatestWindowsTargetPlatformVersion)' != ''">10.0</WindowsTargetPlatformVersion>
   </PropertyGroup>